This invention relates to a process for recovering nickel from aqueous solutions employing nickel tetracarbonyl as an intermediate. More particularly, it relates to a nickel carbonylation reaction which may be conducted at or close to ambient temperatures and pressures.
It is known that nickel can be recovered from its concentrated acidic solutions by electrowinning. However, if the nickel solution is ammoniacal, or if the solution is too dilute, the nickel ions must be extracted by liquid ion exchange and back extracted into a sulfuric acid solution before being electrowon. The purity of the nickel obtained from such a process is very sensitive to the nature and concentration of the impurities present in the solution.
It is also known that nickel can be obtained from solutions by hydrogen reduction at elevated temperatures and pressures. Typically, nickel won by this method is very impure.
Because of the difficulties of the foregoing nickel recovery processes, carbonylation processes for recovering nickel have recently been developed. In the carbonylation process, very pure nickel is obtained by converting nickel values to carbonyl compounds. The gaseous compound Ni(CO).sub.4 is then isolated and thermally decomposed to yield pure metal pellets and carbon monoxide gas. The purity of the nickel metal produced by this process is excellent because of the selectivity of the carbonylation reaction and the fact that other metals often present with nickel are either easily separated or do not form gaseous compounds.
There are many known methods for producing nickel tetracarbonyl from metallic nickel as well as from thermally unstable or reducable nickel compounds such as oxides, formates, acetates, or oxalates. However, the presently known method of carbonylating nickel in aqueous media require extreme conditions, that is, strongly alkaline solutions, high temperatures and pressures, and certain catalysts. For example, in U.S. Pat. No. 3,804,614 to Coffield et al., nickel is produced from ammoniacal slurries of nickel sulfide by reaction with carbon monoxide in the presence of cyanide ion. U.S. Pat. No. 3,775,099 to Coffield et al., in addition to cyanide, discloses that sulfide, cysteine, and tartrate ligands promote nickel carbonylation. U.S. Pat. No. 3,658,475 to O'Neil et al., discloses another carbonylation procedure. However, in practical application, all of the foregoing methods require temperatures above 100.degree. C. and carbon monoxide pressures on the order of 30 or more atmospheres. Thus, in order to produce nickel tetracarbonyl, the carbonylation reaction must be carried out in expensive autoclaves.